Not all invasions go without a hitch. Just ask Nimh.

His invasion should have been easy, straightforward and not overly time exhausting.

Except for the targets of his invasion being superhuman freaks. Now, in order to conquer this not so pathetic world, he's going to need to train. From the bottom of F-Rank, he will need to ascend to the Heights of S Rank and if he's lucky, not die along the way.

Some Invasions just aren't worth the headache.